I found a fitment guide (below pic) on another website and used it to make my decision last month on new tires and wheels . I was able to fit 35"x12.5"x18" Nitto Trail Grapplers and 18"x8.5" ATX Crawl wheels on my leveled FX4 with no rubbing and I have not had to do any trimming. It looks great. *2004-2008 F-150 wheel and tire fitment guide.*On a stock truck without leveling kit you can run 33X12.5" tires if it is a 4WD and 31X12.5" tires if it is a 2WD, using stock wheels. *On a leveled truck(2" kit and larger) you can run 35X12.5" tires on a 4WD and 33X12.5" tires on a 2WD truck, using the stock wheels. *Using aftermarket wheels you can run 35s on a 4WD if the wheels follow these guidelines: -For an 8" wide wheel the backspacing is 5.5 inches. -For and 8.5" wide wheel the backspacing needed is 5-6 inches. -For an 9" wide wheel the backspacing is 4.5-5.5 inches.*If the wheel specs are not listed about you will more than likely have to run 33s or even smaller to avoid too much rubber and trimming. These specs are the basics for tires fitting well but you could run 35s if the specs are very similar but you will face bad rubbing.*For a 2WD truck and after market wheels you can run 33s with wheels that fit withen these specs. -For an 8" wide wheel the backspacing is 5.5 inches. -For and 8.5" wide wheel the backspacing needed is 5-6 inches -For an 9" wide wheel the backspacing is 4.5-5.5 inches.*If the wheel specs are not listed about you will more than likely have to run 31s or even smaller to avoid too much rubber and trimming. These specs are the basics for tires fitting well but you could run 33s if the specs are very similar but you will face bad rubbing.*Remember that these are the basics to running aftermarket wheels and tires, certain tires might not rub but others might because of aggresive sidewalls. *These specs should work for most people and instances but if you are worried about fitment get the tire shop to mount the tire on the wheel and put it on the truck then turn the wheel to full lock in both directions to see and possible problems. If the tires you plan on running aren't 12.5 inches wide you still might be able to run 35s because of that reason, for a 4WD and some aftermarket wheels or 33s for a 2WD. It's best if you can find wheels that are a happy medium between the two backspaces for the 8.5" and 9" wheels because you will face more rubbing if you go for 6" of backspacing on a 8.5" wheel instead of 5.5 inches that's right in the middle. *For those who are curious. The stock wheels(17 and 18") are 7.5 inches wide with 5.5 inches of backspacing. The bolt pattern is 6X135mm.